V Compounded feeds

During the last decades, compounded feeds have been developed extensively to improve animal perfor- mance. They include meals or cakes of appropriate formulation for use as complete feeds for poultry or swine or as complements for specific animal situations (e.g. dairy cows). Compounded feeds are prepared from a large variety of ingredients including cereal grains (e.g. wheat, barley, maize), milling by-products, oilseed meals or cakes (e.g. soybean, rapeseed, sunflower, safflower, cotton), dehydrated alfalfa, root crops (e.g. manioc), animal by-products, fats, and miscellaneous by-products from the food industry such as citrus pulp. These materials are shipped to feed mills, stored in silos, blended according to appropriate formulations, and ground to provide feeds, which can be stored in bulk or in bags. More often, meals are further treated and pelleted, transported and stored in bulk on the farm, in silos, or less frequently in bags.

Basically, the quality of compounded feeds depends on appropriate formulation for speciﬁc animal production and nutritional requirements, and the quality of raw materials, in terms of fungal degradation and the presence or absence of mycotoxins. Cereals used in the manufacture of compounded feeds can

be of relatively poor quality, particularly when they are downgraded from food grade due to fungal growth or damage by weather. Mycotoxins can often be found, also due to the availability of very sensitive analytical methods (Veldman et al., 1992b) (see also Chapter 8).

The bacteriological status of compounded (mixed) feeds is also important. Pathogens may be present in the ingredients as a result of cross-contamination during processing and/or contamination during storage. Salmonellae are the most serious bacteriological hazard.

The UK codes of practice for the storage, handling and transportation of raw materials intended for direct use or incorporation into animal feeds (MAFF, 1995a) and conditions used for the design of processes and premises for the production of feed for livestock (MAFF, 1995b) contain the basic hygiene requirements for the production of Salmonella-free feeds. Application of the HACCP principles is strongly recommended (Butcher and Miles, 1995) and applied in certain countries such as The Netherlands.

A Effects of processing on microorganisms Some major ingredients such as cereal grains, milling by-products, seeds, or other plant products are

used as ingredients without prior heat treatment, thereby introducing microorganisms into the feed mixture. Other plant ingredients, such as oilseeds products, are processed either mechanically (e.g. expeller process) with the generation of a large amount of heat, by solvent extraction, or a combination

267 fungal spores. Animal by-products are processed under high temperature conditions (see Section III).

FEEDS AND PET FOODS

Both oilseed products and animal by-products may be subjected to contamination after processing or during storage, and become a source of various bacteria, including salmonellae, to ﬁnished mixed feeds.

Before processing, ingredients are stored, blended, and ground. During these operations, the lot can become contaminated from ingredients, unclean silos and machinery, dust and the environment. Grinding and adding liquid ingredients results in hot and moist conditions, which may favor bacterial or fungal development.

Pelleting is a basic process in mixed feed manufacturing. It is accomplished by grinding the feed, and forcing it through a die. Usually, but not always, feedstuffs are conditioned by steaming or moistening before pelleting, as this makes it easier to make a ﬁrm pellet, which holds its form (Church, 1979). The heat involved in conditioning and pelleting may reduce vegetative bacteria by up to 1000-fold and, thereby, result in pasteurizing the feed (Stott et al., 1975). Enterobacteriaceae, however, have been recovered from pelleted feeds probably as a result of recontamination (Cox et al., 1988). Whether the pelleting process eliminates pathogens such as salmonellae depends upon feed moisture, time and temperature of conditioning, efﬁciency of heat and moisture transfer in the feed, and the number and thermal resistance of microorganisms (Blankenship et al., 1985; Himathongkham et al., 1996).

Pelleting is followed by rapid cooling and removal of excess moisture in a vertical or horizontal cooling system. Such systems may represent a weak point in the operation since insufficient cooling could allow warm, wet material to be transferred into storage bins or silos, with subsequent microbial growth. Also, the air used for drying may be a source of contamination. Dust and deposits in the cooling system, on the conveyor line and in storage bins or silos may constitute growth niches leading to contamination of the finished mixed feed. Thus, the microflora of mixed feeds reflects the initial flora of the ingredients and the conditions of manufacturing.

Pigmented bacteria of the genera Erwinia and Enterobacter (Enterobacter agglomerans) are com- monly present in cereals and cereal by-products of good quality and are typically found in mixed feeds containing high levels of these ingredients. Other Enterobacteriaceae (Citrobacter, Klebsiella, Serratia, Escherichia, Proteus spp.) and Pseudomonas are present at various levels. Gram-positive species of Staphylococcus, Sarcina, Bacillus, Clostridium, and Enterococcus may be found in lower numbers.

The fungi found will reﬂect those in the raw material unless a substantial heat process is applied. When heat treatments have been applied, or when preservative ingredients have been added, sporulat- ing Bacillus and Clostridium species dominate the microﬂora. Fungi are found in low numbers (e.g.

< 2 10 cfu/g) in mixed feeds (IAG, 1993).

B Spoilage The reduced moisture content and low a w help preserve the mixed feed and prevent microbial growth.

Moistened or rehydrated feeds, however, may spoil rapidly due to bacterial and/or fungal growth. Such conditions may result from insufﬁcient cooling and drying in the ﬁnal stages of processing. More often moistening of feedstuffs is a consequence of diurnal variation in temperature during storage in farm silos resulting in water migration and condensation, creating wet spots, which permit bacterial and fungal growth, spoilage, and toxin production.

The spoilage microﬂora of mixed feeds is characterized by the development of bacteria and molds present in low numbers in good-quality feeds. Important bacteria include Micrococcus, Staphylococcus, Bacillus, Streptomyces, and Thermoactinomyces. Mold spoilage during storage is usually initiated by xerophiles including Eurotium species and Aspergillus penicilloides. As a w increases, other Aspergillus spp. may develop, together with Penicillium spp. If heating occurs, Asp. candidus, Asp. ﬂavus, and later Thermoascus spp. may develop (Sauer et al., 1992; Ominski et al., 1994, see Chapter 8). At higher a w values Paecilomyces, Scopulariopsis, Trichoderma, Chaetomium, Mucor, and Rhizopus may develop.

MICROORGANISMS IN FOODS 6

C Pathogens Documented evidence demonstrates that mixed feeds may be a source of salmonellae in broiler or swine

production (Edel et al., 1967; Smeltzer et al., 1980; Hinton et al., 1987; Eld et al., 1991; Jones et al., 1991). In some instances, the presence of salmonellae in mixed feeds is traceable to the ingredients used. The importance of contaminated animal by-products has been discussed in Section III.C. Vegetable ingredients have normally been considered to be contaminated to a lesser extent although several investigations have implicated these materials as an important source of salmonellae. Thus, salmonellae were isolated from sunflower and soybean cakes at rates of 22% and 16%, respectively (Mackenzie and Bains, 1976). Imported soybean, sunflower, and rapeseed cakes have been found positive for salmonellae at rates of 3.1, 10.1, and 1.9%, respectively (Jardy and Michard, 1992). Such a frequency of contamination is of particular concern because plant ingredients are used in larger proportions than animal by-products, i.e. up to 50% for soybean by-products; up to 20% for rapeseed or sunflower by- products; 5–6% for animal by-products (if they may be used); 1% for fish meal. The UK information also indicates the importance of oilseed by-products as a source of Salmonella.

The pelleting process has been found to be effective for reducing salmonellae in feeds (Eld et al., 1991). However, in one survey, salmonellae were isolated from mash feeds at a rate of 35% and from pelleted feeds at a rate of 6.3%, indicating that the effectiveness of the pelleting process has to be validated or that recontamination needs to be prevented (Jones et al., 1991).

If salmonellae survive pelleting, they can contaminate the rest of the feed mill. If they become established, and even multiply, a focus of contamination will result. The cooling system is an important source of salmonellae when wet surfaces exist because these surfaces offer an excellent substrate for growth. As in the case of animal by-products, dust and feed deposit along the conveyor line and in silos can become niches of contamination for the feed. In Sweden, during 1983–1987, 94 (40.3%) of 233 strains of salmonellae were isolated from dust and scrapings in processing plants (Eld et al., 1991).

Although mycotoxin formation is possible in compounded feeds after manufacture (as discussed earlier), this requires gross abuse and is usually limited to inadequate storage on farms. More serious and widespread problems arise from the use of poor quality cereal grains. In many parts of the world, cereal grains that fail to meet acceptable standards for human consumption are used in animal feeds. Such grain may have deteriorated due to mold growth before harvest relating to wet harvest conditions or geographical factors (See Chapter 8 for further details).

V Compounded feeds